Published on Web 07/17/2009
Cationic Rhodium(I)-dppf Complex-Catalyzed Olefin Isomerization/Propargyl Claisen Rearrangement/Carbonyl Migration Cascade Ken Tanaka,* Eri Okazaki, and Yu Shibata Department of Applied Chemistry, Graduate School of Engineering, Tokyo UniVersity of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan Received May 12, 2009; E-mail:
[email protected] Cascade reactions, which can eliminate a number of reaction steps and reduce hazardous waste, have attracted much attention in current organic synthesis.1 It is well-known that transition-metal complexes are able to activate σ bonds by oxidative addition of those bonds to the transition metal or by an electrophilic substitution pathway;2 π bonds can also be activated through the formation of a complex between an electrophilic transition metal and the π electrons of alkene or alkyne multiple bonds.3 Sequential activation of these bonds by a single transitionmetal complex would allow the development of a novel cascade reaction. For such a process, a cationic rhodium(I) complex is a promising catalyst, as these complexes can catalyze isomerizations4 of alkenes or alkynes via a π-allyl intermediate generated through cleavage of an allylic C-H bond,5 hydroacylations of alkenes or alkynes via aldehyde C-H bond activation,6 and aromatic amino-Claisen rearrangements via π-bond activation.7 As a target for a cationic Rh(I) complex-catalyzed cascade reaction, we focused on a combination of olefin isomerization and the propargyl Claisen rearrangement that has not been realized to date.8 Toste and Sherry previously reported the novel cationic gold(I)-catalyzed Claisen rearrangement of propargyl vinyl ethers to allenic aldehydes.9,10 If an isomerization of readily prepared and stable allyl propargyl ethers to propargyl vinyl ethers followed by the propargyl Claisen rearrangement can be catalyzed by a single cationic Rh(I) complex, this would be useful from a synthetic point of view. Furthermore, the product allenic aldehyde might react with the cationic Rh(I) complex to form another product.11 Here we describe cationic Rh(I)-dppf complex-catalyzed isomerizations of allyl propargyl ethers to allenic aldehydes and dienals (Scheme 1). We first investigated the reaction of allyl propargyl ether 1a in the presence of a cationic Rh(I)-BINAP complex (5 mol %). Pleasingly, the expected olefin isomerization/Claisen rearrangement proceeded at room temperature to give the corresponding allenic aldehyde 2a, and the carbonyl migration product, dienal 3a, was also formed (Table 1, entry 1). Thus, various cationic Rh(I)-bisphosphine complexes were screened in order to realize the selective formation of 2a. Among the ligands examined (entries 1-7), the use of dppf gave 2a in the highest yield (entry 7). The selective formation of 3a could also be realized by increasing the reaction temperature (80 °C) in the presence of the same Rh(I) catalyst (entry 8). Thus, we explored the scope of the olefin isomerization/Claisen rearrangement cascade by using 5 mol % cationic Rh(I)-dppf complex (Table 2). The products were isolated as the corresponding reduced alcohols because of the instability of allenic aldehydes. Electronically Scheme 1
10822
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J. AM. CHEM. SOC. 2009, 131, 10822–10823
Table 1. Screening of Reaction Conditions for Rhodium-Catalyzed Isomerization of Allyl Propargyl Ether 1a
yield (%)a entry
ligand
temp (°C)
conv. (%)a
2a
3a
1 2 3 4 5 6 7 8
BINAP H8-BINAP Segphos BIPHEP dppb dppe dppf dppf
rt rt rt rt rt rt rt 80
100 100 100 100 72 10 100 100
39 67 70 67 56 0 80 0
27 0 0 0 0 0 5 82
a
Determined by 1H NMR.
diverse aryl and sterically diverse alkyl groups could be incorporated at the alkyne terminus (entries 1-5). With respect to substituents at the propargylic position, not only methyl- but also cycloalkyl- and isopropylsubstituted tertiary propargyl ethers could participate in this reaction, affording the corresponding tetrasubstituted allenes in good yields (entries 1-7). Furthermore, tri- and disubstituted allenes could also be obtained in good yields (entries 9 and 10). It is noteworthy that the reactions of both tertiary and secondary propargyl ethers proceeded under mild conditions (room temperature to 40 °C) in comparison with the precedent gold catalysis, which requires elevated temperatures (75 °C) for tertiary propargyl vinyl ethers.9 Phenyl instead of methyl substitution of the alkene moiety was tolerated, although a prolonged reaction time was required (entry 8). The scope of the olefin isomerization/Claisen rearrangement/ carbonyl migration cascade was also examined using the same substrates and rhodium catalyst (Table 3). Interestingly, the isomerizations of tertiary propargyl ethers 1a-h proceeded at 80 °C to yield the corresponding dienals in good yields (entries 1-8), but the isomerizations were terminated at the stage of the corresponding allenic aldehydes in the case of secondary and primary propargyl ethers 1i and 1j (entries 9 and 10). A possible mechanism for the present isomerization is outlined in Scheme 2. In the first step, the olefin isomerization of allyl ether 1 proceeds to give enol ether 5.12 Next, the propargyl Claisen rearrangement proceeds through activation of the alkyne triple bond of 5,13 affording allenic aldehyde 2. At elevated temperature, activation of the aldehyde C-H bond followed by hydrorhodation of the pendant allene furnishes rhodacycle A. Carbonyl migration then occurs, producing conjugated rhodacycle B.14 β-Hydride elimination followed by reductive elimination generates dienal 3. When R3 ) H, allenic aldehyde 2 rather than dienal 3 is generated, 10.1021/ja9038449 CCC: $40.75 2009 American Chemical Society
COMMUNICATIONS Table 2. Cationic Rh(I)-dppf Complex-Catalyzed Isomerizations of
Allyl Propargyl Ethers 1 to Homoallenic Alcohols 4
entry
1
R1
1 2 3 4 5b 6 7 8b 9 10
1a 1b 1c 1d 1e 1f 1g 1h 1i 1j
Ph 4-MeOC6H4 4-ClC6H4 n-Bu Cy Ph Ph Ph Ph Ph
a Isolated yield. with NaBH4.
b
At 40 °C.
c
R2, R3
R4
time (h)
4
yield (%)
Me, Me Me, Me Me, Me Me, Me Me, Me (CH2)5 i-Pr, Me Me, Me Me, H H, H
Me Me Me Me Me Me Me Ph Me Me
23 19 44 17 30 23 15 72 17 72
4a 4b 4c 4d 4e 4f 4g 4h 4i 2jc
74 72 71 69 49 70 81 45 72 65
a
Isolated as allenic aldehyde 2j without treatment
Table 3. Cationic Rh(I)-dppf Complex-Catalyzed Isomerizations of
Allyl Propargyl Ethers 1 to Dienals 3
entry
1
R1
R2, R3
R4
time (h)
3
% yield (E/Z)a
1 2 3 4 5 6 7 8 9 10
1a 1b 1c 1d 1e 1f 1g 1h 1i 1j
Ph 4-MeOC6H4 4-ClC6H4 n-Bu Cy Ph Ph Ph Ph Ph
Me, Me Me, Me Me, Me Me, Me Me, Me (CH2)5 i-Pr, Me Me, Me Me, H H, H
Me Me Me Me Me Me Me Ph Me Me
16 16 16 16 72 16 16 40 15 16
3a 3b 3c 3d 3e 3f 3g 3h 3i 3j
82 76 76 72 43b 77 67 (4:1) 80 (2:1) 0c 0d
a Isolated yield. b The corresponding allenic aldehyde 2e remained in ∼32% yield. Allenic aldehyde 2i was generated in 61% yield. d Allenic aldehyde 2j was generated in 35% yield along with the decarbonylated byproduct. c
Scheme 2
presumably because of reversible hydrorhodation/β-hydride elimination through rhodacycle C. Consistent with this pathway, enol ether 5d and allenic aldehyde 2d were initially observed by 1H NMR spectroscopy at room temperature (eq 1) and 80 °C (eq 2), respectively.15
In conclusion, we have developed the cationic Rh(I)-dppf complexcatalyzed isomerizations of allyl propargyl ethers to allenic aldehydes and
dienals. Future work will focus on using the cationic Rh(I) complex for sequential activation of σ and π bonds in cascade reactions. Acknowledgment. This work was supported partly by Grants-inAid for Scientific Research (20675002, 19028015, and 21 · 906) from MEXT, Japan. We thank Takasago International Corporation for the gift of H8-BINAP and Segphos and Umicore for generous support in supplying a rhodium complex. Supporting Information Available: Experimental procedures and compound characterization data. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) For recent reviews of cascade reactions, see: (a) Chapman, C. J.; Frost, C. G. Synthesis 2007, 1. (b) Nicolaou, K. C.; Edmonds, D. J.; Bulger, P. G. Angew. Chem., Int. Ed. 2006, 45, 7134. (c) Wasilke, J.-C.; Obrey, S. J.; Baker, R. T.; Bazan, G. C. Chem. ReV. 2005, 105, 1001. (2) For a recent review of transition-metal-catalyzed C-H bond activation, see: Kakiuchi, F.; Kochi, T. Synthesis 2008, 3013. (3) For a recent review of π-electrophilic Lewis acid catalysts, see: Yamamoto, Y. J. Org. Chem. 2007, 72, 7817. (4) For a recent review of transition-metal-catalyzed olefin isomerization, see: Tanaka, K. In ComprehensiVe Organometallic Chemistry III; Crabtree, R. H., Mingos, D. M. P., Ojima, I., Eds.; Elsevier: Oxford, U.K., 2007; Vol. 10, p 71. (5) A mechanism via π-allyl intermediates has been proposed for cationic Rh(I)bisphosphine complex-catalyzed isomerizations of allylic compounds. Allyl ethers: (a) Fatig, T.; Soulie´, J.; Lallemand, J.-Y.; Mercier, F.; Mathey, F. Tetrahedron 2000, 56, 101. (b) Hiroya, K.; Kurihara, Y.; Ogasawara, K. Angew. Chem., Int. Ed. Engl. 1995, 34, 2287. Allyl amines: (c) Nova, A.; Ujaque, G.; Albe´niz, A. C.; Espinet, P. Chem.sEur. J. 2008, 14, 3323. (d) Inoue, S.-I.; Takaya, H.; Tani, K.; Otsuka, S.; Sato, T.; Noyori, R. J. Am. Chem. Soc. 1990, 112, 4897. Allylic and propargylic alcohols: (e) Tanaka, K.; Shoji, T.; Hirano, M. Eur. J. Org. Chem. 2007, 2687. (f) Tanaka, K.; Fu, G. C. J. Org. Chem. 2001, 66, 8177. (g) Bergens, S.; Bosnich, B. J. Am. Chem. Soc. 1991, 113, 958. (6) For selected recent examples, see: (a) Coulter, M. M.; Dornan, P. K.; Dong, V. M. J. Am. Chem. Soc. 2009, 131, 6932. (b) Osborne, J. D.; Randell-Sly, H. E.; Currie, G. S.; Cowley, A. R.; Willis, M. C. J. Am. Chem. Soc. 2008, 130, 17232. (c) Tanaka, K.; Shibata, Y.; Suda, T.; Hagiwara, Y.; Hirano, M. Org. Lett. 2007, 9, 1215. (d) Kundu, K.; McCullagh, J. V.; Morehead, A. T., Jr. J. Am. Chem. Soc. 2005, 127, 16042. (e) Sato, Y.; Oonishi, Y.; Mori, M. Angew. Chem., Int. Ed. 2002, 41, 1218. (f) Tanaka, K.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 11492, and references therein. (7) (a) Saito, A.; Kanno, A.; Hanzawa, Y. Angew. Chem., Int. Ed. 2007, 46, 3931. (b) Saito, A.; Oda, S.; Fukaya, H.; Hanzawa, Y. J. Org. Chem. 2009, 74, 1517. (8) For selected recent examples of sequential olefin isomerization/Claisen rearrangement of di(allyl) ethers, see: (a) Kerrigan, N. J.; Bungard, C. J.; Nelson, S. G. Tetrahedron 2008, 64, 6863. (b) Trost, B. M.; Zhang, T. Org. Lett. 2006, 8, 6007. (c) Nelson, S. G.; Wang, K. J. Am. Chem. Soc. 2006, 128, 4232. (d) Nevado, C.; Echavarren, A. M. Tetrahedron 2004, 60, 9735. (e) Schmidt, B. Synlett 2004, 1541. (f) Nelson, S. G.; Bungard, C. J.; Wang, K. J. Am. Chem. Soc. 2003, 125, 13000. (g) Le Notre, J.; Brissieux, L.; Semeril, D.; Bruneau, C.; Dixneuf, P. H. Chem. Commun. 2002, 1772. (h) Ben Ammar, H.; Le Notre, J.; Salem, M.; Kaddachi, M. T.; Dixneuf, P. H. J. Organomet. Chem. 2002, 662, 63, and references therein. (9) Sherry, B. D.; Toste, F. D. J. Am. Chem. Soc. 2004, 126, 15978. (10) For a Ag(I)-catalyzed propargyl Claisen rearrangement, see: Grissom, J. W.; Klingberg, D.; Huang, D.; Slattery, B. J. J. Org. Chem. 1997, 62, 603. (11) A thermal Claisen rearrangement of di(allyl) ethers followed by a Rh(I)-catalyzed intramolecular hydroacylation has been reported. See: Eilbracht, P.; Gersmeir, A.; Lennartz, D.; Huber, T. Synthesis 1995, 330. (12) No reaction was observed upon treatment of 1a with 5 mol % PdCl2 (CH2Cl2, rt, 16 h), which is known to be a very effective catalyst for isomerizations of allyl ethers to enol ethers through electrophilic double bond activation. Therefore, electrophilic double bond activation by the cationic Rh(I) complex might not be involved in this olefin isomerization, although the precise mechanism cannot be determined at the present stage. For a PdCl2-catalyzed isomerization of allyl ethers, see: Mereyala, H. B.; Lingannagaru, S. R. Tetrahedron 1997, 53, 17501. (13) No reaction was observed upon treatment of isolated enol ether 5d in refluxing CH2Cl2 for 14 h. Furthermore, the reaction of 1h with 5 mol % Rh(I)-(S)H8-BINAP in refluxing CH2Cl2 furnished 4h with 15% ee. These results clearly indicate that the cationic Rh(I) complex indeed catalyzes the propargyl Claisen rearrangement. (14) (a) Tanaka, K.; Fu, G. C. Chem. Commun. 2002, 684. (b) Yu, R. T.; Rovis, T. J. Am. Chem. Soc. 2006, 128, 2782. (c) Yu, R. T.; Lee, E. E.; Malik, G.; Rovis, T. Angew. Chem., Int. Ed. 2009, 48, 2379, and references therein. (15) Isolated crude 2a (ca. >90% purity) was indeed transformed into 3a in 72% isolated yield under the same conditions as entry 1, Table 3 [5 mol % Rh(I)–dppf, 80 °C, 16 h].
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